1. Field of the Invention
The present invention relates to a demodulating circuit in a stereo receiver. More specifically, the present invention relates to a demodulating circuit in a stereo receiver adapted such that stereo separation is changed as a function of the electric field strength thereby to improve a signal to noise ratio.
2. Description of the Prior Art
It has been well known that in a stereo receiver a signal to noise ratio on the occasion of a stereo state is worse than that on the occasion of a monaural state and in particular in a weak electric field a signal to noise ratio is considerably degraded as compared with a monaural state. On the other hand, considering an electric wave such as a broadcasting wave, points of a large electric field strength and points of a small electric field strength are at random distributed in accordance with the so called Rayleigh distribution. In particular, considering a case where an electric wave is interrupted by a building, for example, an electric field strength becomes extremely small. Accordingly, in the case of a stereo receiver such as a car stereo set which is borne on a vehicle so that a receiving point is changed from time to time, a signal to noise ratio is changed from time to time in accordance with such distribution of an electric field strength. At a point where an electric field strength is extremely small, mostly a noise component is received and becomes a hardly audible state. Therefore, one conventional approach has been proposed and put into practical use that in the case of an electric field strength smaller than a predetermined level a reception state is abruptly switched forcibly from a stereo state to a monaural state, whereby a signal to noise ratio is improved. However, such approach for instantaneously and forcibly switching from a stereo state to a monaural state involves disadvantages to be described subsequently. More specifically, (1) since a signal to noise ratio is abruptly changed, a discomfortable feeling is given to a listener, (2) since a sound is abruptly changed from a stereo state to a monaural state, a strange feeling is caused to a listener, and (3) another noise is caused incidental to the above described forcible switching from a stereo state to a monaural state.
Therefore, another approach for improving the above described disadvantages has been proposed. One example of such approach is disclosed in U.S. Pat. No. 3,673,342 issued June 27, 1972 to Muller and entitled "CIRCUIT ARRANGEMENT FOR IMPROVING THE SIGNAL-TO-NOISE RATIO OF A STEREO DECODER." A German counterpart of the above referenced U.S. Pat. No. 3,673,342 is German Pat. No. 19 39 422.
Referring to FIG. 1, the above described prior art will be briefly described. Meanwhile, referring to FIG. 1 and further FIGS. 3, 7, 8 and 9, curves shown therein have been illustrated as a line for simplicity of illustration. Therefore, it is pointed out that each of such figures is intended to merely show such curve in a qualitative manner.
Referring to FIG. 1, the curve A shows a signal to noise ratio on the occasion of a monaural state and the curve B shows a signal to noise ratio of a stereo state. As shown in FIG. 2, a signal to noise ratio has a close relation with a stereo separation. More specifically, the smaller the stereo separation the better the signal to noise ratio. Accordingly, by adjusting a stereo separation a signal to noise ratio is improved. Therefore, the above referenced patent is aimed to improve a signal to noise ratio by controlling the same as shown by the curve D in FIG. 1 by controlling the separation as shown as the curve C in FIG. 1. More specifically, according to the prior art of the above referenced patent, a stereo separation is not degraded in the range III of an electric field strength, while a stereo separation is degraded in approximately reverse proportion to an electric field strength in the ranges I and II of an electric field strength. As a result, a signal to noise ratio is changed in accordance with the curve D and thus the above described three disadvantages (1), (2) and (3) are all eliminated. However, the above described prior art still involves a problem to be solved. This will be described in more detail with reference to FIG. 2. Referring to FIG. 2, the ordinate indicates a separation and the abscissa indicates an improvement of a signal to noise ratio in accordance with a change of the separation. As is seen from FIG. 2, the degree of improvement of a signal to noise ratio with respect to a change of a stereo separation is not necessarily in a proportional relation. Accordingly, a decrease in a stereo separation does not necessarily achieve an improvement in a signal to noise ratio. For example, even if the stereo separation is decreased from 50 dB to 40 dB, the improvement in a signal to noise ratio is approximately 0.2 dB at the most. Even if a separation is changed from 40 dB to 30 dB, the improvement in a signal to noise ratio is approximately 0.3 dB. In other words, according to the prior art of gradually decreasing a stereo separation in accordance with an electric field strength when the electric field strength becomes smaller than a predetermined value, as done in the above referenced patent, a situation could occur in the range II of an electric field strength shown in FIG. 1, in which a stereo separation is not good and an improvement in a signal to noise ratio is neither achieved. Therefore, particularly in the range II of an electric field strength, a disadvantage is left that only a separation is degraded although little influence is exerted even in a stereo state, as far as a signal to noise ratio is concerned. Accordingly, while the above described prior art achieves a sufficient effect in improving a signal to noise ratio in the range I of an electric field strength shown in FIG. 1, the same still involves another problem that conversely the separation is degraded in the range II of an electric field strength.